Resonant fiber optic gyroscopes (RFOGs) have better rotation sensitivity for a given diameter then laser ring gyroscopes and interferometer fiber optic gyroscopes. Thus, RFOGs are desirable because a given level of rotation sensitivity can be achieved with smaller RFOGs, reducing the size of inertial navigation systems incorporating RFOGs. RFOGs, however, are susceptible to bias error because of imperfections in optical components, e.g. due to birefringence, which create of one or more parasitic light waves having undesired polarization states. Such parasitic light waves circulate in the RFOG along with the desired light signals, and cannot be distinguished from the desired light signals. Such bias errors vary over time, including due to changes in temperature. Therefore, there is a need to reduce such bias error.